The locust borer, Megacyllene robiniae , is an insect native to North America
and can be found over most of the United States. The adult locust borer is large, nearly
an inch in length, and very colorful. Both males and females have black bodies with bright
yellow markings. These markings are most notable in the "W" on the first pair of
wings. Legs are red and antennae are black. These long antennae place it among the
"long-horned" beetles. These adults are seen predominantly in the fall and can
be found feeding on goldenrod.

Eggs are laid in cracks in the bark of the tree. They hatch and the larvae
bore into the inner bark where they overwinter. This causes bleeding that results in wet
spots on the bark. In the spring, the larvae move into the
sapwood of the tree. Their feeding during this process produces a whitish-colored wood
dust that can be seen on the bark. The gradual enlarging of their tunnels, until they are
3" to 4" long and ¼" to ½" inch wide, moves them into the heartwood
and the wood dust becomes more yellow. With large infestations, this dust may accumulate
at the base of the tree.

By mid-summer, the larvae have matured to the pupal stage. This stage is completed in
about 4 weeks and the adult beetlesthen
exit the tree through the openings made by the larvae. The timing of these stages varies
from one part of the country to another, depending on each area's climate.

The damage inflicted by these borers results in twig and branch dieback, weakening the
tree and leaving it susceptible to wind breakage. Heavily infested trees will be found to
have many broken limbs and knotty swellings on the trunk and larger branches.

Healthy trees, and trees with a greater than 6" diameter, are more resistant to
borer infestation. Trees weakened by drought are especially susceptible to invasion by
borers. These borers attack only black locust (Robinia pseudoacacia) and its cultivars. The honey locust (Gleditsia
triacanthos) is not affected.

Control

Heavily infested trees should be removed and burned to destroy larvae. Borers are
difficult to control with insecticides because they are protected within the trees.
Insecticides, permethrin or carbaryl, are most effective when used as sprays on trunks and
larger limbs during those times when adults are active and laying eggs. Lindane is also
effective but has come to be increasingly restricted and may not be available. These
pesticides should be applied in the spring to control larvae before they bore into the
sapwood, and again from late August through September. This will catch the unprotected
adults, when the female borers are laying eggs, and also protect the trees from newly
hatching larvae.